Thursday, October 14, 2010

Photocathode for Photoinjectors

I've been attending a workshop this week on photocathodes for photoinjectors. With more stringent requirements and more demanding environment that these photocathodes are subjected to (example: higher field gradients, extremely low emittance, etc.), there is a deliberate effort to understand even more the physics of photocathodes and photoemission processes using various materials and processing techniques.

Historically, most of the emphasis has been on getting a stable photocathode, or something with sufficient quantum efficiency (QE) with long lifetimes. Metal photocathdoes have been the workhorse for many photoinjectors (such as synchrotron light sources) because they are relatively easier to fabricate, long life times, and not very fussy. But metals such as copper or niobium have very low QE, and with new demands on producing high brightness electron beam, new materials, or new processing/treatment are being investigated.

What is very exciting now in this area of study is that, there is a new influx of experts from the condensed matter/material science field studying photocathodes specifically for accelerator photoinjectors. This is important because, while there have been such experts scattered around studying these photocathodes, there hasn't been a coordinated effort to get more of these experts in, both with theorists or experimentalists. CM theorists are needed because there are many aspects of the photoemission process that resulted in high QE and low emittance beam that needed to be modeled or explained. Experimentalists are needed because they have a wealth of material characterization knowledge that are needed to study the nature of the surface and the nature of the material, and they provide feedback to theorists to make accurate models. At this workshop, there is a major presence CM theorists and experimentalists, and I think people in both accelerator physics and condensed matter/material science/physical chemistry realize that there's A LOT of work that can be done in the study of photocathodes, even though a lot has already been known.

I'm very excited with this development. As someone who came from condensed matter physics and now working in accelerator physics, I've always realized the importance of these two fields getting together and combining their expertise to solve the various problems in photocathodes. In fact, this issue doesn't just affect the application of photocathodes to accelerator photoinjector. It has a direct consequence to many photocathode applications, such as photodetectors, and this includes things such as high energy physics detector (neutrino detectors) and even night-vision goggles. So the impact of the understanding of a better understanding of the physics can be very wide.